1. Field of the Invention
The present invention relates to a mixer circuit to be used for a frequency converter circuit or a measuring instrument.
2. Related Background Art
As information network systems develop rapidly, the demand for a satellite communication systems has also rapidly increased and the frequency bands used are shifting toward a high frequency region. In such a satellite communication system, a down-converter for converting a high frequency signal to a low frequency signal is required, and demand for a mixer circuit used therefor is also increasing. A conventional mixer circuit uses a diode, a bipolar transistor or a field effect transistor (FET) as an active device. When an FET, a conventional Schottkey gate FET (MESFET) or a high electron mobility transistor (HEMT) is usually used.
A biggest problem in designing the mixer circuit using the FET or the FET mixer circuit is that the gate voltage of the FET significantly swings because a local oscillation signal is a large amplitude signal. The change in the gate voltage causes a change in the mutual conductance g.sub.m of the FET, which in turn causes a change in a gate-source capacitance C.sub.gs or an input impedance. When viewed from an RF signal input terminal, the FET mixer circuit operates with a large change in the input impedance of the circuit as a function of a time corresponding to the swing of the local oscillation signal. As a result, the design is hard to attain and a stable operation is not expected.